1. Field of the Invention
The present invention relates to a digital control type temperature-compensated crystal oscillator capable of compensating at a temperature over a temperature compensation range.
2. Description of Related Background Art
A conventional digital control type temperature-compensated crystal oscillator has a structure such as shown in FIGS. 4A and 4B.
In the crystal oscillator shown in FIG. 4A, a temperature detected by a temperature detector 1 is converted into address data by an address converter 2. Temperature compensation data is read from a memory 3 at an address designated by the address data. If the detected temperature is within a temperature compensation range, the temperature compensation data read from the memory 3 is selected by a changeover circuit 4, and supplied to a D/A converter 5 to be converted into an analog voltage. This analog voltage is applied as an oscillation frequency controlling voltage to a voltage controlled crystal oscillator 6 to perform temperature compensation for the oscillation frequency of the voltage controlled crystal oscillator 6.
The memory 3 stores temperature compensation data at address corresponding to measured temperature data within the temperature compensation range, as indicated within the temperature compensation data store region in FIG. 5A. If an address converted by the address converter 2 is judged by an address discrimination circuit 7 that it is lower than the upper limit address, then an initially set upper temperature compensation data, e.g., 21, is selected by the changeover circuit 4 from an upper limit temperature compensation data register 8, and supplied to the D/A converter 5. If an address converted by the address converter 2 is judged by the address discrimination circuit 7 that it is higher than a lower limit address, then an initially set lower temperature compensation data, e.g., 231, is selected by the changeover circuit 4 from a lower limit temperature compensation data register 9, and supplied to the D/A converter 5. In this manner, if an address is over the temperature compensation range, the temperature compensation is performed using limit temperature compensation data in the memory 3.
In the crystal oscillator shown in FIG. 4B, a memory 3 stores temperature compensation data at address converted from measured temperature data within the temperature compensation range, as indicated within the temperature compensation data store regions in FIG. 5B. If an address converted by an address converter 2 is judged by an address discrimination circuit 7 that it is over the temperature compensation range, then an initially set upper temperature compensation data, e.g., 125, is selected by a changeover circuit 4 from a center temperature compensation data register 125, and supplied to a D/A converter 5. In this manner, if an address is over the temperature compensation range, the temperature compensation is performed using center temperature compensation data in the memory 3.
In the digital control type temperature-compensated crystal oscillator shown in FIG. 4A, as shown by a dot line in FIG. 5A, the upper limit temperature compensation data 21 for all address corresponding to measured temperatures lower than the temperature compensation range in store in the memory 3 outside of the temperature compensation data store region, and the lower limit temperature compensation data 231 for all address corresponding to measured temperatures higher than the temperature compensation range is stored in the memory 3 outside of the temperature compensation data store region. Furthermore, the more the measured data goes away from the temperature compensation data store region, the more the temperature compensation data goes away from an optimum temperature compensation data.
In the digital control type temperature-compensated crystal oscillator shown in FIG. 4B, although the circuit structure is simpler, the center temperature compensation data 125 for all addresses corresponding to measured temperatures over the temperature compensation range is stored in the memory 3 outside of the temperature compensation data store region. It can be said that the oscillator at measured temperatures over the temperature compensation range is not subject to temperature compensation in a practical sense. Furthermore, the temperature compensation data near at the upper and lower limits of the temperature compensation range changes greatly, resulting in frequency jump or skip.